iOS 4.2.1 has been long left unjailbroken but the day has come for the untethered jailbreak to go public, Greenpois0n has released the MacOS client for jailbreaking iOS 4.2.1 on all iDevices.

The advantage of the untethered jailbreak is that now your jailbreak will be permanent, the tethered jailbreak will require you to jailbreak your iDevice anytime you restart / shutdown your iDevice. It won't be soon until redsn0w comes out with the windows client for the jailbreak and hope fully they will take less time making a jailbreak for Apple's upcoming iOS 4.3

Source: Tom's Hardware